There is a need for ultrasonic sensors for use in acoustic positioning systems. It is desirable for such sensors to have reasonably broad frequency response and wide angular response (low directivity), to have output impedance and operating voltage that are not too high, to be rugged, and to be inexpensive to manufacture.
Existing ultrasonic transducers include piezo ceramic transducers, such as the Prowave 400ET080, and electrostatic transducers, such as the Prowave 500ES290. These transducers are described respectively at web pages downloaded from www.prowave.com.tw/english/products/ut/enclose.htm, and from www.prowave.com.tw/english/products/ut/es.htm, on Mar. 13, 2008. There are also ultrasonic transducers using PVDF foil, for example the MSI US40KR-01, described on a web page downloaded from www.meas-spec.com/myMeas/sensors/piezoSensors.asp, on Mar. 13, 2008, and electret microphones with ultrasonic response, such as the Aco-Pacific model 7012, described in a catalog downloaded from www.acopacific.com/acopaccat.pdf, on Mar. 13, 2008.
None of these ultrasonic sensors are completely satisfactory for use in acoustic positioning systems. Ceramic ultrasound transducers generally have relatively small bandwidth, for example 2.5 kHz (Q of 15) for a 40 kHz transducer, although some specially designed transducers with two resonance frequencies have bandwidth as high as 10 kHz. They also tend to be larger than 10 mm in diameter, comparable to or larger than a wavelength in air, so they have relatively narrow angular response. They also have high output impedance, requiring complicated electronic circuits, and they are susceptible to high temperatures, which means they require manual assembly and are relatively expensive to manufacture.
Electrostatic ultrasonic transducers also generally have low bandwidth, less than 10 kHz for a 40 kHz transducer, and are usually larger than 20 mm in diameter, so have narrow angular response. They have high output impedance, and require high voltage, typically hundreds of volts, so require relatively complicated circuitry and consume substantial power and take up a lot of space. They are also susceptible to high temperatures, so require manual assembly.
Ultrasonic transducers using PVDF also have low bandwidth, with Q of 6 to 9, diameter greater than 10 mm and narrow angular response around the z-axis of the cylindrical transducer, and high output impedance. They have relatively low sensitivity, the element is fragile and needs to be protected, and tend to pick up electronic induced noise because of the relatively high exposed surface area. Like electrostatic and ceramic transducers, they are susceptible to high temperatures, so require manual assembly.
Most electret microphones are responsive only up to about 20 kHz. The ones that have ultrasonic response, such as the Aco-Pacific model 7012, are very expensive and are generally used for lab equipment. They also have relatively high output impedance, about 2.2 kilo-ohm, though not as high as the other types of ultrasonic transducers discussed above, and are susceptible to high temperatures.
In spite of these drawbacks, there are acoustic positioning systems on the market that use ultrasound transducers. The systems sold by eBeam and Mimio, described on a web page downloaded from www.e-beam.com/products/complete.html, and by a datasheet downloaded from www.mimio.com/products/documentation/mimiointeractive_datasheet.pdf, both on Mar. 13, 2008, use ceramic ultrasound transducers. The systems sold by Pegatech, described in a datasheet downloaded on Mar. 13, 2008 from www.pegatech.com/_Uploads/Downloads/Specs/MNT/MobileNoteTaker.pdf, and by Navisis, described on a web page downloaded from www.navisis.com/ENGLISH/02_tech/principle_navisis.php?tmenu=02 on Mar. 13, 2008, use PVDF transducers. All of these products digitize handwriting to a series of coordinates, which interact with PC software. An ultrasonic transmitter is placed inside a hand held implement. The transmitter sends electronic signals which are picked up by a receiver, located near a writing area. An infrared signal synchronizes the transmitter to the receiver, using an infrared receiver incorporated into the ultrasound receiver.
MEMS microphones, which use a small and thin silicon membrane manufactured by fabrication techniques used in the semiconductor industry, are a relatively new field of technology which is rapidly gaining in market share. Their advantages include small footprint and height, ruggedness, manufacturing repeatability, and relative immunity to electrostatic and RF interference. But existing MEMS microphones are generally not sensitive to the ultrasonic range. An example is the Knowles Acoustics model SPM0102, described in a web page downloaded from www.knowles.com/search/products/m_surface_mount.jsp, on Mar. 13, 2008.
Memstech microphone model MSM2RM-S3540, described in a datasheet downloaded from www.memstech.com/file/MSM2C-RM-S3540%20Rev%20B.pdf, on Mar. 13, 2008, has a MEMS membrane and base mounted on the inner surface of the front side of a case, over an acoustic port in an otherwise substantially solid surface, with the membrane facing away from the front side, toward the back of the case.
The following patents and published patent applications also describe MEMS microphones: U.S. Pat. No. 6,522,762 to Mullenborn et al, US published applications 2004/046245 and 2002/0102004, both to Minervini, US published application 2007/0071268 and published PCT application WO 2007/022179, both to Harney et al, U.S. Pat. No. 7,301,212 to Mian et al, US published application 2007/205492 to Wang, US published application 2002/0067663 to Loeppert et al, US published application 2004/0170086 to Mayer et al, and published PCT applications WO 2007/018343 and 2007/129787, both to Song.